The amount of intake air into a plurality of cylinders deviates from a target value due to aging deterioration of parts of an intake system and a valve gear system of an internal combustion engine. Errors from such deviation are attributed to mechanical elements and vary from cylinder to cylinder, since the cylinders are uniformly controlled such that a uniform air/fuel ratio is realized, the amount of fuel injection into cylinders does not vary. Although each cylinder receives the same target value, the air/fuel ratio differs from cylinder to cylinder.
Especially under a low load condition such as idling operation, influence of error on the intake air amount increases. And the air/fuel ratio largely varies from cylinder to cylinder, deteriorating the exhaust gas purifying rate of a catalyst, which leads to a lower emission performance. If a cylinder receives excessively lean or rich air/fuel ratio, the cylinder may suffer a misfire. Or, since torque largely varies among the cylinders, drivability deteriorates relative to idling vibration and surging. Therefore, there is a need for a control technique capable of detecting variation of intake air amount among the cylinders and of compensating the variation quickly.
Japanese Patent No. 2689368 discloses an observer control for compensating variation of intake air amount based on output of an LAF sensor (air/fuel ratio sensor) mounted on an exhaust system. According to this technique, however, stability with respect to modeling error and dead time is low, and it is difficult to apply this technique when the response of the LAF sensor varies or aging deterioration is large. It is also difficult to apply this technique to a high output/high torque exhaust system (4-2-1 total exhaust system) which includes a large dead time in measuring the air/fuel ratio.